The role of chloramphenicol's reductive metabolism in chloramphenicol-induced aplastic anemia is being studied. In vitro studies of chloramphenicol was shown to be reduced to amine by rat cecum contents. However, xanthine oxidase failed to reduce chloramphenicol however, it converted nitroso chloramphenicol to N-hydroxy chloramphenicol indicating different enzyme systems for the reduction for chloramphenicol. In in vivo studies nitroso cloramphenicol failed to produce aplastic anemia in mice at doses up to 40 mg/kg i.p.